Electron gun



April 26, 1960 T. M. SHRADER 2,934,666

ELECTRON GUN Filed Dec. 31, 1956 2 Sheets-Sheet 1 fP/Q/O/Z /l/Q?" il?? 11H4 fr TERRY M SHBADER April 26, 1960 T. M. sHRADER 2,934,666

ELECTRON GUN Filed Dec. 3l, 1956 2 Sheets-Sheet 2 /f ff ,DR/01 AR 7 INVENTOR. l TERRY M. SHRADEH 4f l BY f 2,934,666 ELECTRON GUN Application December 31, 1956, Serial No. 632,622 7 Claims. (Cl. 313-84) This 'invention relates to the production of a uniform cross section, high current electron beam and finds particular applicability in high frequency modulator tubes, for use with magnetrons for example. For this reason the invention will be described, by way of example only, as incorporated in a modulator tube. The invention resides in the novel construction of a magnetically focused electron gun including a new and improved magnet pole piece, and is especially concerned with the production of a strong, uniformly convergent magnetic eld in the region of the cathode of the gun.

In an electron gun which uses a magnetic focusing field, a more compact unit can be obtained by embedding the electron gun within a cavity in a pole of the magnet which provides that magnetic eld. In such a unit the magnetic ield is made convergent in the region of the electron gun and tends to focus or converge the electrons into a dense stream or beam as they travel away from the cathode. This type of electron gun, known as a convergent beam gun, has some very special advantages. For example, it is highly desirable for producing high beam current densities since a large electron-emissive cathode area can be used to provide electrons for a relatively small cross section beam.

However, when an electron gun is used which has conventional wire wound grids interposed in the beam path adjacent the cathode, and when the magnetic field within the cavity is no stronger than that provided by prior art practice, serious problems of nonuniformity and low intensity of the beam are encountered. In the absence of a strong magnetic ield in the cavity, the strong electric elds which always exist therebetween the grid wires focus the electrons of the beam into a series of very narrow parallel ribbons. Due to the resultant decreased size of the total actual beam cross section, the electron current which can iiow may be reduced to as much as one tenth of the otherwise obtainable current. In Fig.' l, view a shows a perspective with cross section of a theoretically ideal beam of rectangular shape. View b shows a perspective with cross section of the same shape beam developed as a result of a weak magnetic iield and strong electric fields between the grid wires. This is the type beam obtained by prior art practices. View c shows a perspective with cross section of the same shape beam obtained by use of the present invention. It will be noted that here the narrow parallel ribbons representing the partial cross sections of nearly lill the general outline of the overall cross section represented by view a. As a result the total current of the beam is correspondingly increased and a more uniform cross section provided.

Accordingly, one object of my invention is to obtain a convergent electron beam having the desired uniform composition and configuration of Fig. lc.

To this end another object of my invention is to provide a convergent magnetic focusing eld whose strength is relatively high within the cavity of a magnet pole piece.

These objects are Vaccomplished in my invention by view b have been expanded tov United States PatentA yrated in the ltube of Fig. 1.

particularconguration The making the magnet pole piece as a composite structure, with a body of low reluctance magnetic material and a capping plate of material of somewhat higher reluctance. A cavity having a constricting throat is formed through the higher reluctance plate into the low reluctance body. In the preferred embodiment the higher reluctance plate is made up of alternate laminations of magnetic and nonmagnetic material, with the laminations lying generally perpendicular to the beam axis. This results in a plate having a desired overall reluctance lintermediate that of the magnetic and nonmagnetic materials.

The use of an intermediate reluctance plate .on the face of the pole piece provides a relatively higher reluctance path to magnetic ilux out of the front face than is obtained in a cavitated pole piece of all magnetic material. By so doing, ux spillage is enhanced from around lthe vthroat to within the cavity. After spilling into the cavity, the magnetic ilux lines are bent and attractedto the opposite polarity pole a the far end of the tube. In this way a stronger convergent orv tunneling field is achieved within the cavity.

In the description of the present invention and in the accompanying claims the terms magnetic, and low reluctance are used somewhat interchangeably-and describe a similar property.

Likewise, nonmagnetic, and high reluctance are used interchangeably and describe a similar property. Each of the two pair of terms is used relative to the other to describe relative degrees of permeability. Within these terms of reference, soft iron might comprise amagnetic material and copper a nonmagnetic material.

In the remaining drawings: p Fig. 2 illustrates a simplied modualtor tube incorpojrating my invention.

Figs. 3 and 4 show a front elevation and longitudinal section view, respectively, of a'pole piece'as incorpo- It is one preferred because of its simplicity for rectangular beam applications.

Figs.y 5 through l0 show three other representative forms of cavitied magnet pole pieces which may embody my invention.

Figs. 11a and 1lb illustrate by contrast, respectively, the shapes and relative strengths of the magnetic fields formed by a prior art pole piece and by the pole piece of my invention.

Referring to Fig. 2, the` tube 1 includes a vacuum tight envelope 2 containing at one end thereof an electron 'gun assembly 4 for producing an electron beam 6, at the other end an electron collector 8, and intermediate the gun andcollector an electrode system. The latter may be of any conventional form, for example with parallel electrodes 12 and 14. Also included within the envelope 2 are two magnet pole pieces 16 and 18 for'the purpose of providing the requisite magnetic focusing iield. These may be either permanent magnets themselves or magnetically permeable pieces having associated therewith eitherv an external permanent magnet or a solenoid which produces a magnetic field.

The entire electron gun assembly 4 is housed within 'the pole piece 16, and comprises a unitary electron cage assembly which may include either conventional wire grids or conventional apertured disk electrodes. For example, the electron gun shown has a cathode 20, a heater 16, which is the subject of my invention, may be supported in any conventional manner.

Referring to Figs. 3 and 4, the pole piece 16 is cubical in general external shape. However, it should be understood that for successful practice of my invention it is not necessary that the overall outside shape befof any pole piece 16 consists of `the channel 30 ,ate reluctance laminated front region two-*main parts, a body member 28 of magnetic (low i capping plate 42-44 in flush abutment with the channeled side of the body member 28. The capping plate is made up of alternate laminations of magnetic material 32 and nonmagnetic material 34. The magnetic material'of both the body 28 and the plate 42-44 may be soft iron while the nonmagnetic material may be copper. A cavity 36 is provided in the composite pole piece 16 which opens from through the external face of the laminated plate. The cavity 36 is in the form of a dovetail-like channel and provides opposed tapered walls 38 and 40 on the plate 42-44 to assist in shaping the magnetic field. `Such a pole piece may be fabricated from three parts; a horizontally, rectangularly channeled body member .28, and two laminated end plates 42 and 44 of trapezodial cross section. The shorter of the two parallel g sides of each of the two laminated end plates is affxedin abutment with one of the faces of the channeled side of the body member such that a constrictedl entrance to the cavity is provided. An aperture 46 may be provided in the rear of the body member toenable lead wires to be brought into the electron gun elements. However, if the cavity is left with open sides then this Vaperture is not necessary since lead wires may be brought in from the open sides.

Figs. 5 and 6 show a front elevation and longitudinal section view, respectively, of a pole piece 47 identical to that of Figs. 3 and 4 with two exceptions: (1) the recess or channel 48 extends only across the central region of the pole piece thus providing a cavity 49 with side walls and (2) the intermediate fabricated from a solid homogeneous piece of a material such as an alloy possessing the characteristic of the desired intermediate reluctance. l

Figs. 7 and V8 show corresponding views of a pole piece 51 identical to that of Figs. 3 and 4except that its overall outside shape iscircularly cylindrical.

Figs. 9 and l0 illustrate corresponding views of -a pole piece 52 having a circular throat opening 53 adapting it for use where a beam of circular cross-section is desired.

Here the cavity 54 has the shape of a right circular cylinder beveled on one end. This results in a convergent magnetic lield with 360 symmetry rather than the planar lsymmetry about a horizontal center line as in the pole pieces of Figs. 3 through 8. The pole piece 52 also illustrates two modifications which may be incorporated either singly or in combination in this or any other of the embodiments of my invention. Here, the intermedi- 55 is provided by slotting an otherwise unitary pole piece of magnetic'materiaL As such, the slots S6 are composed of the medium surrounding the pole piece, for example, a vacuum, and serve as the nonmagnetic lamnae. As a further modication the laminae are convergent forward toward the center rather than being perpendicular to the direction of beam ow.

Referring now to Fig. 11a, a cavitied pole piece 57 is shown which consists of all magnetic material constructed according to the prior art. Most of the flux lines 58 are conducted around the cathode 59 and out through the front face 61 with very few spilling out of the body of the pole piece and into the cavity 63. However, the fact that flux lines are mutually repulsive results in their tendency to spread out by pushing away from each other. This fact in itself will cause some few lines to traverse a path out the pole piece 57 and into the cavity 63.

In the pole piece 16 according to my invention, as shown in Fig. 1lb, the addition of an intermediate reluc- `tance capping plate l2-4l4 on one face of the low re- .luctance portion 2870i the pole piece causes the spillage intofthe cavity to be more pronounced, giving a strong convergent or funneling eld within the pole piece. This I result obtains from the fact that the ux has two Aalternareluctance capping plate 50 is,

tive paths which it can follow: out of the front face 64, or into the cavity 36. The addition of the capping plate 42-44 causes an unbalance in the relative reluctances of these alternative paths. The flux will then redistribute itself, and some of the lines that otherwise would have followed the path out of the front face will be diverted along the alternative path into the cavity. This action may be likened to an electrical circuit in which the resistance in one of two parallel paths is increased-the analogous result being an increase of current in the other path. It should be here noted that in Figures 11a and 1lb the lines of force representation are meant to illustrate only a qualitative comparison and not a quantitative one.

Experimental results show that' whereas a Iield of approximately gauss inside the cavity was obtainable with prior art pole pieces, it is possible to approximate a 1500 gauss field inside the cavity by using a pole piece according to my invention. Such an increased field strength within the cavity prevents the electron beam from being formed into the narrow parallel ribbons as described in Fig. lb.

In accordance with my invention the capping plate may simply be constructed of a solid piece of intermediate reluctance material as shown in Figs. 5 and 6. However, I have found that a laminated construction of alternate magnetic and nonmagnetic laminae oiers some special advantages. In a laminated construction it is possibleto conveniently obtain a desired predetermined reluctance of the capping plate by preselecting the relative thicknesses of the magnetic and nonmagnetic lamnae. Moreover, if the laminations are oriented transverse or generally perpendicular to the direction of ux ow, the capping plate exhibits a higher reluctance in the direction out through the front face of the pole piece than it does in the general direction toward the cavity. As may well be appreciated from the foregoing explanation in reference to Figs. 11a and 11b, such a differential reluctance characteristic will act to enhance spillage.

I claim:

l. A magnet pole piece comprising a body member of low reluctance magnetic material and a plate of intermediate reluctancevoverlaid on one'face of said body member, said pole piece having a cavity therein opening through said plate, said cavity havingwall portions defined by said plate which are convergent toward the outer face of said plate to form a constricting throat in the region of said outer face.

2. A magnet pole piece for producing a convergent magnetic eld comprising a body member of magnetic material having a recess therein and a laminated plate of alternate magnetic and nonmagnetic laminae having a tapered apetrure therethrough, said plate having its side of larger aperturein lush abutment with the recessed face of said body member, said Vaperture being substantially axially aligned with the opening of said recess and substantially identical in shape to the cross section of the beam to be produced, whereby the aperture of said laminated plate combines with the recess of said body member to form a throated cavity adapted to house an electron gun.

3. A beam tube including a magnet pole piece comprising a body member consisting of a block of low reluctance magnetic material having a cavity recess in one face thereof and a plate of intermediate reluctance with a rectangular aperture therethrough having at least two of its opposite sides convergently tapered, said plate having its larger aperture opening side in ush abutment with said recessed face, the larger opening of said aperture being substantially the same size as said recess and axially aligned therewith, and an electron gun mounted in the cavity formed by said recess and aperture for projecting a convergent beam through said aperture.V

4.k The beam tube according to claim 3,'` wherein said 5 plate of intermeidate reluctance comprises alternate laminae of magnetic material and nonmagnetic material.

5. In a beam tube adpated to use a rectangular-shape electron beam, a magnet pole piece for providing a convergent magnetic ield to shape said beam comprising a block of magnetic material and a laminated plate of alternate magnetic and nonmagnetic laminae aixed to a face of said block in ush abutment, said pole piece having a dovetail channel therein formed through said plate and into said block` having a constricting throat of relatively higher reluctance than that of said block, said channel providing a cavity adapted to house an electron 6. In a beam tube adapted to use a circular convergent electron beam, a magnet pole piece for producing a convergent magnetic iield having 360 symmetry about the axis of the beam comprising a body member. of magnetic material, a laminated plate of alternate magnetic and nonmagnetic laminae overlaid on one face of said body member, said pole 'piece provided with a throated cavity opening through the surface of said laminated plate and adapted to house an electron gun, said cavity having the shape of a right circular cylinder beveled on one end.

7. A mag-net pole piece comprising a irst portion of low reluctance magnetic material and a second portion of somewhat higher eiective reluctance next to said first portion, said pole piece having a cavity therein opening through said second portion and including wall portions defined by said second portion which are convergent outwardly to form a constncting throat for said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 

